1. Field of the Invention
This invention relates to circuit breakers and more particularly to a circuit breaker with an electronic trip unit which responds to sputtering arc-type faults.
2. Background Information
Conventional residential circuit breakers have a thermal trip device which responds to persistent overcurrents of moderate magnitude to provide a delayed trip and a magnetic trip device which responds instantaneously to overcurrents of large magnitude. Thus, the fault current must reach a predetermined magnitude, for example, ten times rated current, for the instantaneous trip to occur, or the overcurrent must sustain a predetermined average value over a given time interval to implement the delayed trip.
There is a type of fault, however, which may not produce either the peak magnitude required for the instantaneous magnetic trip, or the sustained average overcurrent necessary for the delayed trip, yet it may pose a fire hazard. This is the intermittent or sputtering arc-type of fault. Such a fault can occur, for instance, between two conductors that are in close proximity, but not touching, so that an arc is struck between the conductors. This arc can produce a temperature high enough to melt the copper in the conductor. The melted droplets of copper can ignite flammable materials in the vicinity. However, the resistance of the wiring may be high enough to limit the peak current and the ac current cyclically passes through zero to extinguish the arc so that the average current is low. Thus, the conventional circuit breaker does not respond to the fault, although a hazard exists. This is especially true in the case of a stranded wire extension cord where an individual strand can be melted at a relatively low fault current.
As sufficient voltage is required to strike the arc of a sputtering arc fault, this type of fault typically occurs at the peak of the ac voltage waveform thereby resulting in a step increase in current. Switching of some residential loads also produces step increases in current. For instance, an iron which is turned on at the peak of the voltage waveform results in a step increase in current; however, the magnitude of the step is less than the rated current of the circuit breaker. In addition, inrush currents, such as those produced by the starting of a motor, also rise rapidly, although not as rapidly as an arc-type fault. Furthermore, inrush currents tend to decay exponentially while faults maintain a constant high value, or drop to zero in the case of a sputtering arc.
There is a need for a circuit breaker which can respond to a sputtering arc fault.
There is also a need; however, for such a circuit breaker which can distinguish between acceptable inrush currents and overcurrents produced by sputtering arc faults.
There is an additional need for such a circuit breaker which is reliable and economical.